Postage stamp with $6$ and $7$ cents question What is the largest postage in cents that cannot be paid exactly with an unlimited supply of $6$-cent and $7$-cent stamps? Any hint so that I can proceed?

You are interested in solving the coin (Frobenius) problem for $n =2$ and coins $6, 7$. Although there is no general answer, in case $n = 2$ there is one: because $6$ and $7$ are coprime, the result will be $6 \cdot 7 - 6 - 7 = 29$.